(A.) Field of the Invention
The present invention relates to thin-film sputter-deposition techniques to form cermets and more particularly to radio-frequency (RF) sputter deposition to form cermets having multiple, ultra-thin layers of electrically conductive and insulating materials.
(B.) Description of the Prior Art
A cermet (ceramic-metal) is an immiscible composite of an electrically conductive and an electrically insulating material. Cermet materials typically have good abrasion and corrosion resistance, they can withstand temperatures of 100.degree.-200.degree. C. without any significant changes in their physical properties, and their resistivity can be tailored by the proper choice of the metal/insulator ratio and the microstructure of the material. Due to the above physical characteristics, this class of materials is a suitable candidate for hybrid circuit elements where high stability and reliability is required. In the past, cermet materials have been made by sintering a mixture of a metal and an insulator powder at high temperatures. Presently, thin-film techniques have replaced some of these methods because of better accuracy in the resistivity of the material, improved reproducibility, and enhanced stability and reliability.
Thin-film techniques of making cermets include: (1) coevaporating a metal and an insulator simultaneously; (2) sputtering from a composite metal/insulator target; and (3) cosputtering from two different targets, a metal and an insulator target respectively. Each of these methods, including their respective advantages and disadvantages, will be discussed hereinbelow.
Coevaporation entails the evaporation of a metal and an insulator simultaneously, usually from two different sources. While the evaporation of a metal is a rather routine procedure, the evaporation of the insulator is not. During evaporation of insulating compounds, the compounds tend to decompose, thus resulting in poor quality control of the thin-film material.
It is well-known in the art that sputtering can be used to deposit high-quality films. Since highresistivity materials cannot be sputtered using DC methods, the following discussion will be confined to radiofrequency (RF) sputtering methods. In composite sputtering of a cermet, a single target is usually manufactured by pressing together a composite of the two components desired in a predetermined ratio. Alternatively, a piece or pieces of a first material are mechanically fixtured or imbedded in a target of a second material. An advantage of composite sputtering is that both materials can be sputtered from a single target, and the entire film may be formed in a single step. A disadvantage of composite sputtering is that it is difficult, if at all possible, to change the ratio of the conductor and insulator once the composite has been made. Therefore, one must know in advance what composition is required for the desired degree of resistivity. Thus, composite sputtering is not versatile. Furthermore, the sputtering rate of the two components in the composite target typically will be different; thus producing a film with a nominal composition which is different from that of the starting material. In addition, a composite target will often contain impurities (for instance, carbon, commonly introduced during hot-pressing) that may affect the thin film properties.
As previously noted, another method for depositing composite films involves cosputtering in which two plasmas are induced at the same time from two separate targets. Samples sit on a rotating table that passes under each target for a certain amount of time determined by the rotation speed of the table. Two major advantages of cosputtering are: (1) it is relatively versatile; and (2) the relative amounts of conductive versus insulating material can be changed by changing the power on the respective targets. Another advantage of cosputtering is that reasonably good uniformity of composition may be achieved through the film thickness. A disadvantage of the cosputtering technique is that the composition of the film produced by this method cannot be specified with great accuracy. Another disadvantage is that the grain size of each component in the film cannot be altered without changing sputtering parameters such as voltage or pressure of the sputtering gas, which will affect both targets. Another disadvantage of cosputtering is that nonlinear interactions between the two plasmas may affect the reproducibility of thin films manufactured by this method. Yet still another disadvantage of cosputtering is that one of the targets cannot be sputtered reactively (if needed) without affecting the other target.